1. Field of the Invention
This invention relates to a method and circuitry for suppressing transient disturbances in a data channel and, more specifically, to a method and circuitry for suppressing transient disturbances arising from collision between a magnetoresistive sensor and aspersed particles adhering to a magnetic disk surface.
2. Description of the Related Art
In a data channel for a magnetoresistive (MR) sensor, a transient disturbance can result from a "thermal asperity". When a hard particle trapped on the surface of a magnetic disk collides with a MR sensor riding closely adjacent to the disk surface, a rapid temperature rise occurs in the sensor. This friction-created temperature increase of up to 120.degree. C. first occurs at the point of contact between particle and MR sensor. The localized temperature increase produces a small but sudden increase in temperature of the entire MR sensor; perhaps as much as several centigrade degrees averaged over the whole sensor, within 50 to 100 nanoseconds. Because the MR sensor has a non-zero temperature coefficient of resistance (e.g. 0.003/C..degree. for permalloy), the sensor resistance then increases in response to the sudden temperature rise.
The heat conducted into the MR sensor from the localized hot spot diffuses slowly from the sensor to the local environment, causing the typical delayed exponential decay known for such thermal asperities. For instance, the increased sensor resistance can be seen to decline about 30% within the first one-half to five microseconds following collision.
Because the MR sensor detects magnetic signals by exploiting the magnetoresistive effect, resistance changes arising from magnetic changes on the disk surface adjacent to the sensor are detected as changes in voltage across the sensor. A DC bias current induces the voltage across the sensor resistance that varies according to changes in the sensor resistance. Thus, a thermal asperity induces a superimposed voltage transient on the desired data signal from the sensor. Because MR sensor non-linearity increases with increasing magnetic signal excursions about the sensor bias point, the sensor is designed to keep the magnetic excursions induced by desired data signals sufficiently small to ensure reasonable sensor linearity. For instance, detection of a magnetic change on the disk surface may require only a 0.3 percent change in sensor resistance. Thus, thermal asperity transients can exceed 400 percent of the typical base-to-peak magnetic data signal voltage amplitude from the MR sensor.
The high amplitude and long decay time of a thermal asperity transient severely disrupts the data stream from a MR sensor, perhaps masking 5 to 30 bytes of contiguous data pulses. Such a long series of errors is very difficult to correct using normal error correction codes. If a sensor pulse detection circuit uses automatic gain control (AGC), thermal asperity transients can disrupt even longer sequences of data because of increases in transient decay time arising from AGC settling time effects.
Practitioners in the art have proposed two fundamental types of solutions to the thermal asperity transient problem: improved error correction codes and voltage transient cancellation schemes. Both types of solutions tend to be complex and expensive to implement.
In U.S. Pat. No. 4,914,398, Steven A. Jove et al disclose a method and circuitry for cancelling thermal asperity transients in a magnetoresistive sensor channel by pulling the voltage transient quickly back to zero. Their solution is useful but requires significant additional circuitry and leaves imperfections in the signal waveforms such as increased additive correlated noise and spurious pulses.
P. W. Chung et al ("Pre-filtering In The Design of Peristaltic Envelope Detectors", IBM Technical Disclosure Bulletin, Vol. 33, No. 10B, pp. 48-52, March 1991) propose a switched pre-filter that modifies the MR sensor detection circuit frequency response to remove elements of the thermal asperity transient signal. Their front-end pre-filter distorts the desired data signal but may remove noise that cannot be properly cancelled with a transient cancellation scheme such as taught by Jove et al above. Thus, the Chung et al solution is ultimately even more expensive and complex than that suggested by Jove et al.
In U.S. Pat. No. 4,916,701, John S. Eggenberger et al disclose a method for correcting multi-byte errors in a data channel coupled to a magnetic medium on which data is recorded in variable length blocks. The blocks must include error correction code (ECC) for which ECC "syndromes" are generated during reading. While their method is very useful for correcting long error bursts, it is expensive in terms of data storage space and error correction processing resources and is incompatible with many data formats. In combination with thermal asperity transient cancellation methods known in the art, the Eggenberger et al ECC method offers a useful means for correcting errors created by thermal asperity transients. However, the resulting combination of methods is complex and expensive, requiring substantial additional components, data storage and data processing resources.
There is a clearly felt need in the art for a simpler and more efficient method for overcoming data errors in a MR sensor channel arising from thermal asperity transients. This unresolved problem is solved by this invention in the manner described below.